A solar cell converts solar energy directly to DC electric energy. Generally configured as a semiconductor photodiode, a solar cell permits light to be absorbed in the semiconductor resulting in generation of charge carriers (electrons and holes) which are then extracted as electrical current. Usually, photodiodes are formed by combining p-type and n-type semiconductors to form a p-n junction.
Electrons on the p-type side of the junction within the electric field (or built-in potential) tend to be attracted to the n-type region (usually doped with phosphorous) and repelled from the p-type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration (e.g., phosphorous, arsenic, antimony, boron, aluminum, gallium, etc.) often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
In general, typical solar cells are formed on a silicon substrate doped with a first dopant (the absorber region), upon which a second counter dopant is diffused using a gas or liquid process (the emitter region) completing the p-n junction. After the addition of passivation and antireflection coatings, metal contacts (fingers and busbar on the emitter, and pads on the back of the absorber) may be added in order to extract generated charge. Emitter dopant concentration, in particular, must be optimized for both carrier collection and for contact with the metal electrodes.
Referring now to FIG. 1, a simplified diagram of a traditional front-contact solar cell is shown. In a common configuration, a counter doped emitter region 108 (n-type or p-type) is first formed on a lightly doped silicon substrate 110 (p-type or n-type respectively). Prior to the deposition of silicon nitride (SiNx) layer 104 on the front of the substrate, residual surface glass formed on the substrate surface during the deposition process) is substantially removed (PSG in the case of POCl3 or H3PO4, and BSG in the case of BBr3) commonly by exposing the silicon substrate to hydrofluoric acid (HF). The set of metal contacts, comprising front-metal contact 102 and back surface field (BSF)/back metal contact 116, are then sequentially formed on and subsequently fired into silicon substrate 110.
The front metal contact 102 is commonly formed by depositing an Ag (silver) paste, comprising Ag powder (about 70 wt % to about 80 wt % (weight percent)), lead borosilicate glass (frit) PbO—B2O3—SiO2 (about 1 wt % to about 10 wt %), and organic components (about 15 wt % to about 30 wt %). After deposition the paste is dried at a low temperature to remove organic solvents and fired at high temperatures to form the conductive metal layer and to enable the silicon-metal contact. During the firing process, as the temperature is increased up to about 400° C., the frit softens and forms a molten glass which wets and dissolves the underlying anti-reflective coating (e.g., silicon nitride) barrier layer 104 layer in an exothermal redox reaction. During the cooling phase the glass solidifies and silver crystallites tend to form in the layer. These silver crystallites have been shown to form ohmic contact to the underlying silicon and allow conduction through the insulating glass layer.
The formation of these crystals is strongly dependent on the firing temperature and on the underlying doping of the silicon. If the firing temperature and/or dopant concentration is too low then not enough crystals form and the contact has high resistance. If the temperature is too high then the crystals may become too big, penetrate the junction and form an alternative path for current to flow instead of through the external load. This is commonly referred to as a shunt resistance. Over firing (i.e., too hot for too long) may also result in the formation of a glass layer that is too thick. In this case a high resistance contact is formed as the carriers are unable to pass easily through the glass and into the silver electrode. The need to form a glass layer of appropriate thickness with the correct size and number of crystallites creates a narrow process window which compromises the efficiency. In addition to this problem, the presence of glass frit within the metal finger may reduce the conductivity of the finger, adding to the overall series resistance of the solar cell.
BSF/back metal contact 116 is generally formed (from aluminum in the case of a p-type wafer), and is configured to create an electrical field that repels and thus minimizes the impact of minority carrier rear surface recombination. In addition, Ag pads [not shown] are generally applied onto BSF/back metal contract 116 in order to facilitate soldering for interconnection into modules. During the firing process it is often beneficial to use higher temperatures for longer times since this will incorporate more dopant atoms to form the BSF. However, the need to correctly fire the front silver contacts often compromises the ideal firing conditions.
In view of the foregoing, there is a desire to provide optimized methods of forming a metal contact on a silicon substrate.